Semiconductor devices and integrated circuits fabricated on semiconductor substrates often include silicide structures serving as conductive electrodes. In some applications such as, for example, electrostatic discharge protection circuits, input/output drivers, etc., high resistance silicide structures are beneficial. In other applications such as, for example, inverters, low noise amplifiers, etc., low resistance silicide structures are beneficial in improving the circuit performance. To form high resistance titanium silicide structures, titanium is disposed on the semiconductor substrate. The substrate then goes through an annealing process at a temperature not exceeding 700 degrees Celsius (.degree. C.), during which titanium interacts with silicon on the substrate to form silicide in an orthorhombic base centered crystal phase referred to as C49. C49 crystal phase silicide has a resistivity between approximately 60 micro-ohm-centimeter (.mu..OMEGA.-cm) and approximately 90 .mu..OMEGA.-cm. Low resistance silicide structures can be formed in several ways. In one approach, the substrate with high resistivity C49 silicide formed thereon is put through a rapid thermal annealing process at a temperature of between 800.degree. C. and 1000.degree. C. The high temperature annealing transforms the titanium silicide from C49 phase to an orthorhombic face centered crystal phase referred to as C54, which has a resistivity between approximately 12 .mu..OMEGA.-cm and approximately 20 .mu..OMEGA.-cm. In another approach, the low resistance silicide structures are formed through a refractory metal, e.g., molybdenum, implantation, titanium deposition, and annealing. During the annealing process, the titanium interacts with silicon to form silicide structures in the C54 crystal phase because the molybdenum lowers the barriers for phase transformation from the high resistance C49 crystal phase to the low resistance C54 crystal phase.
In some applications, it is beneficial to have both high and low resistance silicide structures in the same circuit element, or in different circuit elements within the same functional block. It is sometimes also beneficial for process integration to have silicide structures of different characteristics formed on the same semiconductor wafer.
Accordingly, there exists a need for a semiconductor structure that includes heterogeneous silicide structures on a semiconductor substrate and a method or a process for forming such a structure. It is desirable for the process to be simple and efficient. It is also desirable for the process to be compatible with existing semiconductor device fabrication processes.